These polysulphides, which have found diverse industrial applications, in particular as extreme pressure additives and as sulphidation agents for catalysts, are at present prepared industrially by the reaction of a mercaptan with liquid sulphur in the presence of a basic catalyst. In spite of a fairly high temperature (130.degree.-140.degree. C.), the reaction EQU 2RSH+(n-1)S .fwdarw.RS.sub.n R+H.sub.2 S
is often incomplete. The products obtained are turbid and contain mercaptan and free sulphur; they also contain a large proportion of heavy S.sub.6, S.sub.7, S.sub.8 . . . polysulphides which are unstable and steadily deposit sulphur.
The use, as catalyst, of the combination of a mercaptan with an alkene oxide and an alkali metal base (French Pat. No. 2,607,496, incorporated by reference) leads to a practically complete reaction, but noticeably increases production costs.